1. Field of the Invention
The invention relates to a stator structure for a fan motor with an outer rotor, and more particularly to a radial winding stator structure for a single phase motor.
2. Description of the Related Art
FIGS. 1A and 1B are a top view and an A—A cross-sectional view showing a conventional radial winding stator 1 for a fan motor with an outer rotor, respectively. In the four-pole stator, only the magnetic flux lines of force produced by a permanent magnet (not shown) of the rotor and linked with stator coils that contribute to the rotational torque of the fan motor, so the portions of an electrically conductive wire 2, which is wound around the pole bobbins 12 of the four-pole radial winding stator 1 and protrudes over a pole surface 11 in an axial direction cannot contribute to the rotational torque of the motor. That is, bending portions 21 of the wire cannot contribute to the rotational torque of the motor.
Consequently, with regard to the conventional four-pole radial winding stator 1, when the number of turns of the wound electrically conductive wire 2 is increased in order to provide a larger magnetic force, the ratio of the electrically conductive wire bending portions 21 protruding over two ends of the pole surface 11 in the axial direction to the total electrically conductive wire is getting greater and greater, thereby causing more and more ineffective regions. Consequently, the efficiency of the four-pole radial winding stator structure of the fan motor cannot be effectively improved.
On the other hand, FIG. 1C shows a silicon steel sheet 3 constituting the radial winding stator 1. The silicon steel sheet 3 is composed of a hub 31 and poles 32 extending from the hub 31. Each pole 32 is formed with a bobbin 321 and an end portion 322. The external surface of the end portion 322 is a camber having an arc length S1 determined by θ1 and R1, wherein θ1 denotes an angle of each end portion 322 on the overall circumference, and R1 denotes a distance from a center of the hub 31 to the external surface of the end portion. Please refer to FIGS. 1A to 1C simultaneously. Since the stator is composed of several stacked silicon steel sheets 3, the space occupied by the bending portions 21 in the axial direction in the overall stator is getting larger as the bending portions 21 of the electrically conductive wire protruding over two ends of the pole surface 11 are getting more, thereby adversely influencing the number of silicon steel sheets that may be stacked within a fixed height H of the stator. Consequently, the magnetic flux lines of force of the permanent magnet (not shown) of the rotor cannot be completely conducted.
Also, in a fan motor structure, there is a settled limitation to the height of the radial winding stator. With regard to the conventional four-pole radial winding stator 1, the number of silicon steel sheets can not be further increased because the bending portions 21 of the electrically conductive wire protruding over two ends of the pole surface 11 in the axial direction are too large. For example, in FIG. 1B, the height L1 of the silicon steel sheets and the heights x of the electrically conductive wire protruding over two ends of the pole surface 11 are limited by the threshold height H of the stator. Thus, it is impossible to enhance the efficiency of the fan by increasing the number of silicon steel sheets.